Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

Abstract

Abstract. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed highenergy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.

A statistical study on O<sup>+</sup> flux in the dayside magnetosheath

“…Observations of O + in the dayside magnetosheath during storm conditions were presented by Zong and Wilken (1998) and Zong et al (2001), who suggested the ions to be of ring-current origin, and referred to both flow along open field lines and finite gyroradius effects as possible escape mechanisms. Dayside observations of O + escape close to the equatorial plane correlated with a northward IMF were reported by Kasahara et al (2008), who showed that the escape took place along open field lines in relation to reconnection processes. Hirahara et al (1997), Marcucci et al (2004) and Taktakishvili et al (2007) are other examples of observations and studies of O + escape.…”

Structure and Dynamics of the Magnetopause and Its Boundary Layers

“…It has been shown that in their usual concentration (∼5%), alpha particles (He ++ ) in the solar wind and magnetosheath are probably not the primary cause of the subunity Walén slope (Puhl-Quinn and Scudder, 2000). On the other hand, oxygen ions (O + ) of magnetospheric origin, if abundant, may significantly affect the actual Alfvén speed (e.g., Kasahara et al, 2008).…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…Therein, a significant fraction of cusp ions are sufficiently accelerated to escape through the magnetopause with a total escape flux of ∼7 · 10 24 s −1 . Possible processes responsible for the loss at the dayside are reconnection with the northward directed IMF (Kasahara et al 2008) or (2006) with observations of O + intensity at >274 keV measured by Cluster/RAPID in the plasma sheet at AE < 150 nT and AE > 300 nT, respectively the southward directed IMF , drift losses (mainly triggered by the solar wind dynamic pressure which changes the magnetopause shape; this is also applicable to the nightside plasma sheet) and due to the interaction with Pc 5 waves when the ion kinetic energy drops below the level required to overcome the convection potential, wherefore the ion is lost at the dayside magnetopause by sunward drift (Li et al 1993;Yang et al 2011). Supermagnetosonic plasma streams can lead to the escape of O + from the magnetosphere (Savin et al 2014).…”